The present invention relates generally to optical systems including a light source and a photodetector where the photodetector provides an output signal having an intensity proportional to the amount of light received by the photodetector. More particularly, the present invention relates to an automatic blanking circuit for such an optical system to automatically compensate for variations in intensity of the light source caused by age, deterioration or even dirt on the light source or the photodetector. The present invention may be utilized in connection with any one of a number of optical systems including but not limited to systems employed in clinical laboratory equipment where absorbance of light by a sample, or transmittance of light through a sample, or reflectivity of light from a sample is detected by the photodetector. Outside the clinical laboratory field, the present invention may be utilized in optical systems where absorbance, reflectivity, or opaqueness is being measured such as, for example, optical readers and optical character recognition systems.
Since one use of the present invention is as a part of a spectrophotometer in connection with measuring the concentration of a substance in solution and, more specifically, for determining hemoglobin species in a blood sample, the invention will be explained in that context. Such explanation, however, should not be construed as limiting the applicability of the automatic blanking circuit.
By way of further background, when measuring the concentration of a substance such as hemoglobin in solution, the concentration of the substance measured photometrically usually follows Beer's Law where the negative logarithm of the transmittance varies as a function of the concentration. Specifically the formula is log (I/I.sub.o)=-KC, where the ratio (I/I.sub.o) is referred to as the transmittance of light, i.e., the light which is not absorbed by the sample. In the formula, the transmittance, I/I.sub.o is the ratio of the intensity of a beam of light passing through the solution divided by the intensity of the same beam of light passing through a "blank" solution, i.e., a solution containing a zero concentration of the substance to be measured. Thus it is important that the "same" beam of light be used for both measurements, i.e., there must be no aging or degradation of the optical system between measurements.
In order to achieve the desired results, it is initially necessary to determine the transmittance of a zero concentration solution. According to the prior art, this is usually done by inserting a vial containing a blank or zero concentration solution, often water, into the path of light between the light source and a photodetector, and adjusting the output to 100% transmittance. This adjustment may be accomplished in several ways according to the prior art. First, the intensity of the light source itself may be varied. Second, the intensity of the amount of light in the light path to the solution may be varied by a diaphragm or variable slit. Third, a 100% transmittance value can be achieved by varying the sensitivity of the photodetector. Finally, since the ultimate output is usually determined by an indicator such as a meter, the 100% transmittance value can be achieved by varying the internal resistance of the meter, through a conventional potentiometer, so that the meter reading is 100% transmittance when a blank vial is inserted in the path between the light source and the photodetector.
There are many disadvantages in each of the above techniques in that they all require manual adjustments by the operator of the equipment. In addition, the basic shortcoming of each of these techniques is that each fails to account for aging or variations in the intensity of the light source or dirt in the optical system. While a potential solution to this problem has been suggested in U.S. Pat. No. 4,128,339, such solution still requires a multiple step manual calibration followed by a computer operation in an attempt to provide an accurate transmittance value. In the system of the aforementioned patent, if the calibration occurs once each day, any aging of the light source or dirt in the optical system between calibrations will still not be accounted for and erroneous readings will result.
The present invention overcomes these problems by providing a blanking circuit which automatically compensates for any degradation in the optical system.